Flux switching motors are characterized by an unwound, salient pole rotor and two sets of fully pitched windings on the stator. One of these sets of windings, the field, carries substantially unidirectional current. The other set, the armature, is excited by bidirectional current, the polarity of which is determined by the rotor position.
Flux switching motors may be advantageously used in a variety of applications involving large household appliances and power tools such as table saws, mitre saws and other tools requiring greater than a fractional horsepower output. Flux switching motors are also highly advantageous for use in power tools such as saws because of the lack of brushes and the conventional commutator that is used with universal motors. The lack of brushes and mechanical contact between the brushes and a commutator allows a sealed motor to be constructed which is highly immune to dust and dirt which could otherwise affect operation of the brushes and commutator of a conventional universal motor. Such a motor also has a longer life and is much less likely to require periodic repair and/or maintenance because of the lack of wear and tear that would normally be present when a commutator and brushes are required for commutating the motor.
With flux switching motors, it has been common to commutate such motors electronically through the use of a pair of electronic switches. The switches are controlled via some form of a controller in such a manner that the direction of current flow through one of one or more armature windings, or through different portions of a bifilar armature winding, can be controlled to commutate the motor.
Many such conventional commutation circuits have required the use of a “snubber” circuit to provide a path for current flow as the electronic switches are switched off and commutate the motor. Such a snubber circuit, however, has to dissipate a fair amount of power, which represents wasted power, each time current is switched through one of the armature windings or through portions of a single bifilar winding. The copper utilization of such a scheme is also very low.
Excitation circuits for present day flux switching motors also typically require an aluminum electrolytic capacitor to be included across the output of the rectifier portion of the circuit to create a steady dc voltage and to handle the transients created while commutating the motor. However, without the aluminum electrolytic capacitor, typically referred to as a “bulk” capacitor, starting of a flux switching motor from rest may be very slow and non-uniform. Additionally, without such a bulk capacitor, it can take an unacceptably long time for the motor to reach its operating speed. In many applications, such as with power tools such as table saws or mitre saws, it would be undesirable for the user to have to wait several seconds or more before the motor reached its operating speed before the user could be able to use the tool.
Such bulk capacitors, however, also contribute to a low power factor, typically 0.75-0.70, which reduces the power that the motor can draw from a current protected branch circuit. Bulk capacitors are also relatively large and take up a fair amount of space on a printed circuit board, in addition to having life constraints (typically about 2,000 hours). They also are prone to failure from vibration, and therefore are not especially well suited to use in power tools. Still further, bulk capacitors can not mitigate the effects of harmonics into the AC source. While this is presently not a serious consideration in the United States, the introduction of harmonics into an AC source in Europe is a very serious consideration and one factor that must be considered when designing an excitation circuit for a motor to be used in Europe.
It would therefore be highly desirable to provide an excitation circuit for a flux switching motor which provides for the recirculation of current through the armature winding using an arrangement of a plurality of electronic switches and a switching control scheme to electronically commutate the motor. It is a related object to eliminate the need for a conventional snubber circuit through the use of the just-described switching control scheme and arrangement of switches.
It is still another object of the present invention to provide an excitation circuit for a flux switching motor which makes use of a relatively small, film capacitor across the output of the rectifier portion of the excitation circuit, rather than the traditional bulk capacitor. The use of a film capacitor, rather than the traditional aluminum electrolytic capacitor, would significantly improve the power factor of the circuit in addition to significantly reducing harmonics that might be introduced back into the AC source by the circuit. It would also positively contribute to the mitigation of EMI (Electro-magnetic interference).
It is still another object of the present invention to provide an excitation circuit for a flux switching motor which makes use of a switching circuit which can be controlled to effect reverse commutation of the armature winding of the motor, and thus bring the motor to a quick stop when the motor is turned off. Such a feature would also be highly desirable when a flux switching motor is used in various power tools such as table saws, mitre saws, rotary hammers, etc.
Still another consideration when any form of electric motor is used with a power tool such as a saw, drill, sander, router, etc., is inadvertent starting of the power tool if the user has his/her finger on the on/off switch (e.g. the on/off trigger) when the tool is initially plugged in to an AC power source. In this instance, if the user is not aware that he/she is engaging the on/off switch while plugging a power cord of the tool into an A C power outlet, the sudden starting of the motor could startle the user. While an electronic controller for electronically commutating the motor can be used to monitor for the position of the on/off switch of the tool when AC power is first applied, it would be even further desirable to provide a separate switch monitoring circuit for such a purpose. Accordingly, if the electronic controller were to malfunction and not sense that the on/off switch was in the “on” position when AC power is first applied to the power tool, the independent switch position monitoring circuit would still be able to detect this condition.